the opening guitar chord that seems to be tuning down. It sounds similar to a slide guitar, but how on earth do you get this effect in software? I understand you could tune it down but this sounds more seamless and suiting.
The pitch-slide does sound like it was created by the [fret-less ?] guitar rather than in post-production.
Audacity comes with a "sliding time-scale/pitch-shift" effect.
The bending pitch of the electric guitar chord in the beginning is created with the handle on the electric guitar, by which one can regulate the tension of all strings combined. Thus, if you play a chord and then move the handle, all the tones go up or down with the same pitch change.
In the youtube video you can hear that it's done by hand. A computer-edited pitch slide would sound as if the pitch is sliding down at the same rate but this is not the case here, it sounds like the guitar player is adjusting on the spot.
Maybe this thing can be approached by using Audacity, select the part of the chord that you want to slide, and put the pitch slide effect over it (if that's the right English word).
Yes definitely 'whammy bar' or tremolo arm....as heard on many Pink Floyd recordings.